This invention generally relates to a simple door latch which provides leverage to the door in both the closing and opening operations of the latch's door handle which is particularly desirable for ultra-low temperature (ULT) freezers. The latching handle of the invention, when rotated by a user, applies a closing force to compress a door gasket and an opening force to unseat a frozen gasket or some other resisting force. Doors that have frozen onto their gaskets and therefore make the door difficult to open are a particular problem associated with ULT Freezers.
ULT freezers generally operate at temperatures from −70° C. to below −145° C. The door gasket seal may account for 30% or more of the heat leak into the cabinet. Proper compression of the gasket seal is imperative to obtain a seal that exhibits good thermal resistance to the relatively warm ambient air. It is desirable that this be accomplished by a latch that requires an operating effort no more than can be offered by a person of minimum strength. This implies that the door latch should not require the application of a substantial force against the door in order to get the latch engaged and that the door be closable and latchable by a one hand operation.
When a ULT freezer cabinet door is opened, ambient moisture tends to condense onto the door gasket and the cabinet's thermal breaker assembly. Under certain conditions, when the door is subsequently closed, the condensed moisture will freeze and bond the door gasket to the frame of the cabinet and thus require a large force to break the bond and re-open the door. In the prior art, heating has been provided in the door frame either electrically or by a warm refrigerant line. This works most of the time but occasionally doors become frozen to the door frame and therefore require applying an external opening force of substantial magnitude often through application of a prying bar.
A similar problem arises when the door needs to be opened shortly after being closed. In this situation, the ambient air that entered the freezer when the door was open becomes cold. The reduced air temperature in the freezer cabinet reduces the pressure within the cabinet causing a force of considerable magnitude across the door that tends to hold the door closed. A vacuum breaker is generally provided to ameliorate this problem of differential pressure. However, the equalization of pressure afforded by the vacuum breaker takes a little time, perhaps up to 10 minutes. Anyone who needs access to a recently opened and closed freezer must therefore wait until the internal pressure has been equalized with the ambient pressure. It would be of great utility if a simple door latch were provided to assist in opening the door in cases where it is frozen shut or is held closed by differential pressure or both.
During closing, the door gasket must be properly compressed to obtain good thermal resistance to the ambient air. The compression force applied by the gasket against the door is large and therefore substantial leverage is needed to allow a person of minimum strength to apply the necessary force. This leveraged force should also be reversible in order to more easily open the door in the event that the door is adhered to the gasket by condensed water that has frozen or in the event that the internal cabinet pressure has dropped to a low level thereby creating a large force that resists opening.
Another desirable feature of a latch for a ULT freezer is that the freezer door latch can be moved by a simple single handed operation to unlatch and pull open the door and similarly to push closed and latch the door. The reason is that commonly a user has an object in one hand when opening or closing the freezer door such as a container of vials to be placed in or just removed from the freezer.
It is therefore an object and purpose of the invention to provide a door latching mechanism that applies a sufficient gasket compression force without requiring more effort than can be provided by one handed operation of a person of minimum strength and also can be leveraged open by the same one handed effort as is available from such a person.